Thermocouples are used to measure temperature by converting thermal energy into a change in electrical resistance at a junction between two specially selected materials. This change in resistance allows an electronic measurement of the temperature of a thermal region of interest based upon the electrical resistance of the thermocouple junction. The thermal region of interest may be, for example, (a) a thermally sensitive region on the surface of a piece of equipment, (b) a temperature monitoring location for a material being thermally processed, or (c) an environmentally sensitive region in a body of fluid.
It is often desirable to use thermocouples to measure temperatures in an environment that includes microwave energy, such as in a microwave oven, or in an environment that includes other radio frequency energy, such as in a traveling wave tube or other electronic device. Hereinafter the terms “radio frequency field(s)” and “RF field(s)” will be used to refer to electromagnetic fields (EMF) that range from approximately 9 kHz to several thousand GHz—a range that includes both conventional “radio” frequencies and microwave frequencies. Also, the terms “radio frequency range” and “range of radio frequencies” will be used to refer to the same range of frequencies: approximately 9 kHz to several thousand GHz. Applications of thermocouples in RF fields at conventional microwave thermal processing frequencies (e.g., 915 MHz and 2.45 GHz) are of particular interest.
As will be appreciated by one of skill in the art, either a bare thermocouple (thermocouple junction with lead wires) or a thermocouple probe (a bare thermocouple encased in a jacket) may be used as a “thermocouple device” to measure temperatures. When measuring temperatures in an RF field, the bare thermocouple or the thermocouple probe tends to concentrate electrical fields which in turn perturbs the RF field. This concentration of the electrical fields may result in arcing and/or heating of the thermocouple junction, the probe or the probe tip. This heating may cause the thermocouple device to register a temperature that is higher than the actual temperature of the target subject. Also, because of this heating, it is not uncommon for the thermocouple device to fail at the thermocouple junction when placed in a high energy RF field. What are needed therefore are designs that overcome these and other deleterious effects of RF fields that are experienced by conventional thermocouple devices.